The present invention relates to the configuration of an optical passive network system in which a plurality of subscriber apparatus share an optical transmission line, and to an operation method for the system, and more particularly to a system configuration and operation method suitable for system extension such as elongation of a transmission distance and an increase in the number of subscriber apparatus.
Because the demand for communications using broadband is so high, access lines of users are changing to large capacity access lines using an optical fiber instead of using techniques of telephone lines such as Digital Subscriber Lines (DSLs). From the viewpoint of line laying and maintenance management cost, widely used as an access network system is a Passive Optical Network (PON) system (hereinafter called simply PON, or an optical passive network system or a passive optical network system where appropriate). PON is a system in which an optical signal is transmitted and received between a station side apparatus and a plurality of subscriber apparatus, through branches and multiplex using optical fibers and optical splitters. The station side apparatus is hereinafter called an optical line termination (OLT) or a parent station. The subscriber apparatus is hereinafter called an optical network unit (ONU) or a child station. Standardization (recommendations) of the PON system is being made by various standardization facilities, e.g., International Telecommunication Union (hereinafter called ITU-T (International Telecommunication Union-Telecommunication Standardization Sector), and introduction to each country is prevailing. For example, introduction of Gigabit capable PON (GPON) stipulated by ITU-T Recommendations G984.3 starts already in each country.
General home subscribers hereinafter called users in some cases) increase the chances of communications for information collection and social life by accessing the Internet or the like, and an increase in the number of access networks is requested for making users connect a communication network. In order to increase the number of users, it is considered to additionally introduce PON itself to be used as an access network, i.e., to add an OLT, or to increase the number of ONUs accommodated by OLT of PON. PON is generally configured in such a manner that OLT performs all works including complicated system control such as bandwidth control and management of accommodated ONUs. OLT is therefore far more expensive than ONU. A cost incurred for newly laying optical fibers is a large expense for the carrier. A desired solution method is therefore to increase the number of accommodated ONUs per OLT, rather than to additionally install an OLT. An already existing PON has a limit in its performance such as an attenuation amount of an optical signal propagating in an optical fiber to be caused by a transmission performance of the optical fiber and the number of branches of an optical splitter. Many of GPONs have settings of a maximum communication distance of 20 km and a maximum number of 64 of branches of the optical splitter (the number of ONUs connectable to OLT). In this connection, studies have started on a relay (hereinafter called Reach Extender (RE)) aiming at elongating a communication distance of an optical fiber and increasing the number of branches. The basic concept of the studies is to realize elongation of a communication distance of an optical fiber and an increase in the number of branches by properly installing RE in an optical signal communication section between OLT and ONUs and to make OLT control RE. As its control protocol, ITU-T Recommendations G984.6 was stipulated using an ONU Management Control Interface (OMCI): an already existing ONU control protocol. Introduction of RE allows high speed Internet access services to be provided in the district with delayed prevailing of IT technology other than the central urban district. Attention is paid to RE, as one access network prevailing spread method.
A method of introducing RE to PON includes a method of inserting RE into a trunk fiber (also called line concentrator optical fiber) used commonly by each ONU between OLT and an optical splitter, and a method of inserting RE into a branch line optical fiber used between an optical splitter and each ONU.
As RE is inserted into a line concentrator optical fiber, a communication distance is elongated more than a conventional PON. Subscriber ONUs existing in a remote district can therefore be accommodated in the same OLT so that it becomes easy to increase the number of accommodated ONUs. Namely, an ONU accommodation efficiency of OLT is improved.
On the other hand, as the communication distance is elongated, a communication time (transmission delay time) to ONU in a remote district prolongs and the number of remote ONUs increases, resulting in prolongation of a signal wait time at ONU and an increase in a signal processing load on OLT. There is therefore a possibility that a communication time assigned to each ONU is shortened. More specifically, in order for OLT to receive a signal from an arbitrary ONU through time division multiplexing, OLT executes a process (hereinafter called Dynamic Bandwidth Assignment (DBA)) of dynamically assigning a communication bandwidth by considering a transmission bandwidth request received from ONU. In this case, since RE is inserted, an expected wait time at each ONU continues to prolong until a transmission permission is acquired after a transmission request is sent. A communication interception (standby) time prolongs at each ONU. Namely, since the communication interception (standby) time prolongs at each ONU in operation, the signal quality requiring real time performance is adversely affected, and the communication capacity (bandwidth) assignment process by DBA for all ONUs becomes complicated, or an assigned capacity is reduced.
It is therefore desired to provide PUN capable of suppressing the above-described issues including an increase in a signal processing load on OLT, prolongation of a signal transmission wait time/reduction in a transmission signal bandwidth at each ONU, and deterioration of a transmission signal quality, PON being desired to provide a communication quality of the same degree as that of a conventional PON, even if RE is introduced to PON and a communication distance between OLT and ONU is prolonged and the number of accommodated ONUs is increased. More specifically, it is an object of the present invention to provide a PON and a PON control method capable of suppressing the above-described issues even if DBA necessary for PON is executed.
Even with the method of inserting RE into a branch optical fiber, ONU performing communications via RE is also associated with the same issues described above. It is characteristic, however, in that other ONUs are not influenced by REs so that a conventional PON control method (DBA) can be used for the other ONUs. It is therefore an another object of the present invention is to provide a PON and a PON control method, PON having ONUs connected via REs and ONUs without REs in a mixed manner and configured in such a manner that a conventional PON control method and a control method incorporating REs are selectively used in accordance with a layout of REs.